Flexible thermoelectric devices offer unique advantages, including mechanical conformability and suitability for wearable and Internet of Things energy harvesting. However, their integration with low-cost polymer substrates requires the low-temperature synthesis of high-performance thermoelectric materials. In this study, impurity-doped polycrystalline Ge thin films were fabricated via solid-phase crystallization at low temperatures (<500 ℃), and their microstructure and transport properties were systematically optimized by controlling the dopant concentration and deposition temperature. As a result, both P-doped n-type and Ga-doped p-type Ge films achieved record-high power factors of 3180 μW·m⁻¹·K⁻² and 1210 μW·m⁻¹·K⁻², respectively, establishing the highest performance reported to date among polycrystalline, environmentally benign thermoelectric materials. The flexible devices demonstrated stable power generation, achieving maximum power densities of 0.70 mW·cm⁻² in the cross-plane configuration, which represent the highest output characteristics among eco-friendly flexible thermoelectric systems. These results establish low-temperature solid-phase crystallization of doped Ge thin films as a promising route to next-generation flexible thermoelectric devices.